1. Field of the Invention
The present invention relates to particle beam metrology and more particularly to an arrangement for improving the precision of critical dimension measurements of objects, for example integrated circuit wafers, using an particle beam device such as a scanning electron microscope.
2. State of the Art
It is known to use electromagnetic systems in microscopes such as scanning electron microscopes (SEM) for measurement and inspection purposes. Scanning electron microscopes are often used in place of traditional optical microscopes for microelectronics inspection and metrology applications in semiconductor manufacturing. The metrology tools are often used, for example, for measuring patterns (e.g., critical dimensions) formed on semiconductor wafers during fabrication.
The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes can achieve resolutions from about 100 .ANG. to 200 .ANG., while the limiting resolution of optical microscopes is typically about 2,500 .ANG.. Further, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes. Despite the accuracy and precision of present scanning electron microscopes, enhanced instrument specifications and capabilities are required as parameters (e.g., critical dimensions) to be inspected come within the submicrometer ranges.
An article entitled "Microelectronics Dimensional Metrology in the Scanning Electron Microscope", Parts I and II, Solid State Technology by Michael T. Postek et al. (November 1986), describes a typical SEM wafer inspection instrument. As described therein, a focused electron beam is scanned from point to point on a specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter are optimized for the specific application and specimen composition.
As the scanning electron beam contacts the surface of a specimen, backscattered and/or secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting these backscattered and/or secondary electrons. A point by point visual representation of the specimen is obtained on a CRT screen as the electron beam controllably scans the specimen.
Although known scanning electron microscopes are able to provide a resolution adequate for semiconductor manufacturing, several factors limit their resolution, or precision. For example, harmonic distortion introduced during signal acquisition and signal processing limits the precision of conventional SEMs to about 10-20 nm. Precision as used herein is defined in terms of the standard deviation of repeated measurements of a particular feature. In particular, precision is defined as six times the standard deviation, or 6.sigma..
Accordingly, it would be desirable to provide an SEM or similar device with the capability of improved measurement precision. Particularly in the manufacture of VLSI devices, since projected technological advances are expected to lead to even further reduced geometries, increased precision is necessary as compared to the precision presently available in the known art. The present invention reduces harmonic distortion introduced during signal acquisition and signal processing, so as to improve by a factor of several times measurement precision.